1. Field of the Invention
The present invention relates to a catalyst for the selective reduction of nitrogen oxides, e.g. NO, NO.sub.2 (hereinafter referred to as NO.sub.x) present in exhaust gases in the presence of ammonia.
2. Description of the Prior Art
Nitrogen oxides (NO.sub.x) are toxic to the human body. When NO.sub.x is absorbed by the human body, body functions begin to decline. There is a very demanding need for efficient methods of removing NO.sub.x because NO.sub.x is an oxidant which causes photochemical smog. Accordingly, it is of utmost importance to remove NO.sub.x from exhaust gases which are evolved from polluting sources such as power plants, nitric acid plants, automobiles and the like. Heretofore, various methods for removing NO.sub.x have been proposed. In the conventional methods, the catalytic reduction method in which ammonia is used as a reducing agent has been effective for treating exhaust gases containing relatively high concentrations of NO and O.sub.2.
In another method, the exhaust gases are contacted with a vanadium oxide catalyst. However, when the vanadium oxide catalyst is prepared by immersing a known carrier in an aqueous solution of a vanadium salt and then calcining the product, and then is used and the NO.sub.x in the gas is reduced in the presence of ammonia, the catalytic activity is not satisfactory and also the optimum temperature range is limited. When the catalyst is used for removing NO.sub.x in a combustion exhaust gas containing NO.sub.x and sulphur oxides, it is hard to maintain the catalytic activity for decomposition of NO.sub.x for a long time.
A need, therefore, continues to exist for an improved method of catalytically reducing nitrogen oxides in exhaust gases.